Method and apparatus for varying signals transmitted by a tag

ABSTRACT

An apparatus ( 10, 240, 300 ) includes a signpost ( 11, 241 - 256, 322, 612, 623, 626 - 628, 652, 661, 682, 686, 703 ) which transmits signpost signals ( 24 ) that are received by a tag ( 12, 271 - 275, 301 - 316, 395 - 397, 616 - 618, 641643, 653, 656 - 657, 662 - 664, 679, 708, 711 ). The tag in turn transmits radio frequency beacon signals ( 72 ) which are received by a reader ( 13, 261, 319, 521 - 530 ) The tag can vary the duration of the beacon signals, for example in dependence on whether it is currently receiving a signpost signal. Further, the tag can dynamically vary the transmission rate and/or transmission power of the beacon signals, for example following receipt of a signpost signal. Varying these parameters can facilitate compliance with governmental regulations.

STATEMENT REGARDING COPYRIGHT RIGHTS

[0001] A portion of this patent disclosure involves material which issubject to copyright protection. The copyright owner has no objection tothe facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the Patent and Trademark Officepatent file or records, but otherwise reserves all copyright rightswhatsoever.

TECHNICAL FIELD OF THE INVENTION

[0002] This invention relates in general to techniques for trackingmobile items, and more particularly, to a method and apparatus fortracking items using radio frequency identification tags.

BACKGROUND OF THE INVENTION

[0003] According to an existing technique for tracking mobile items, adevice known as a tag is mounted on the item, and communicates by radiofrequency signals with a central receiver, which is commonly known as areader. Systems of this basic type have been generally adequate fortheir intended purposes, but have not been satisfactory in all respects.

[0004] In this regard, the manner in which a tag transmits radiofrequency information can create issues relating to compliance withgovernmental regulations, because governmental regulations often effecta balancing between factors such as transmission length, transmissionpower and transmission rate. In existing tags, the tag design involvesselection of a predetermined balance between transmission duration,transmission power, and transmission length, and then operation of thetag is carried out using this predetermined balance.

SUMMARY OF THE INVENTION

[0005] From the foregoing, it may be appreciated that a need has arisenfor a method and apparatus for tracking items using radio frequencyidentification technology, in a manner which permits variation ofcertain characteristics of the transmitted signals. According to a firstform of the present invention, a method and apparatus are provided toaddress this need and involve: receiving in a receiver section of a tagwireless signpost signals that each include a signpost code; andtransmitting from a transmitter section of the tag wireless beaconsignals which each include a beacon code associated with the tag. Thetransmitting activity includes: causing the transmitter section to beresponsive to receipt by the receiver section of a respective signpostsignal for including in at least one beacon signal the signpost codefrom the received signpost signal; and causing the transmitter sectionto transmit the beacon signals in a selected one of first and secondformats which are different, the transmitter section using the firstformat in response to receipt of one of the signpost signals and usingthe second format in response to the absence of receipt of any of thesignpost signals for a specified time interval, the first formatincluding a signpost field containing the signpost code from the mostrecently received signpost signal, and the second format lacking thesignpost field and being shorter in length than the first format.

[0006] A different form of the present invention involves: receiving ina receiver section of a tag wireless signpost signals that each includea signpost code; and transmitting from a transmitter section of the tagwireless beacon signals which each include a beacon code associated withthe tag. The transmitting activity includes: causing the transmittersection to be responsive to receipt by the receiver section of arespective signpost signal for including in at least one beacon signalthe signpost code from the received signpost signal; and causing thetransmitter section to be responsive to receipt by the receiver sectionof one of the signpost signals for automatically effecting variation ina predetermined manner of at least one of a transmission power level anda transmission rate for the beacon signals.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram of an apparatus which embodies featuresof the present invention, and which includes a signpost, a beacon tag, areader, and a control system;

[0008]FIG. 2 is a diagrammatic view of a digital word which istransmitted by the signpost of FIG. 1;

[0009]FIG. 3 is a diagrammatic view of two different digital words,either of which can be transmitted by the beacon tag of FIG. 1;

[0010]FIG. 4 is a diagram showing a sequence and timing with which thebeacon tag of FIG. 1 transmits beacon signals;

[0011]FIG. 5 is a flowchart showing in a different form the beaconsignal sequence which is depicted in FIG. 4;

[0012]FIG. 6 is a high-level flowchart showing still other aspects ofthe operation of the beacon tag of FIG. 1;

[0013]FIG. 7 is a diagrammatic top view of a system which represents onepractical application for an apparatus of the type shown in FIG. 1;

[0014]FIG. 8 is a diagrammatic top view similar to FIG. 7, but showing asystem which represents another practical application for an apparatusof the type shown in FIG. 1;

[0015]FIG. 9 is a diagrammatic perspective view of one type of containerwhich can be used in association with the invention, and which bearsthree beacon tags of the type shown in FIG. 1;

[0016]FIG. 10 is a diagrammatic top view of an installation whichrepresents one example of a practical application of a system of thetype shown in FIG. 1;

[0017]FIG. 11 is a diagrammatic view of selected portions of a systemwhich embodies the invention and which is suitable for use inassociation with the installation of FIG. 10;

[0018]FIG. 12 is a diagrammatic view of a train which includes atractor, three trailers, and a container on each trailer, and whichembodies certain aspects of the present invention;

[0019]FIG. 13 is a diagrammatic side view of a forklift that carries twosignposts of the type shown in FIG. 1, a ceiling bearing several beacontags of the type shown in FIG. 1, and several items carried by theforklift which each bear a beacon tag of the type shown in FIG.

[0020]FIG. 14 is a diagrammatic side view of the tail section of anairplane, and a loader which can be used to load or unload the airplane;

[0021]FIG. 15 is a diagrammatic side view of an apparatus which includesa conveyor, a signpost of the type shown in FIG. 1 that is mounted abovethe conveyor, and several items that are traveling along the conveyor ona palette, and that each have thereon a beacon tag of the type shown inFIG. 1; and

[0022]FIG. 16 is a diagrammatic sectional side view of an apparatuswhich is an alternative embodiment of the apparatus shown in FIG. 7, inthat it includes the addition of a sensor which can affect the operationof the signpost

DETAILED DESCRIPTION OF THE INVENTION

[0023]FIG. 1 is a block diagram of an apparatus 10 which embodiesfeatures of the present invention. The apparatus 10 includes a signpost11, a beacon tag 12, a reader 13, and a control system 14. The apparatus10 actually includes many signposts of the type shown at 11, many tagsof the type shown at 12, and several readers of the type shown at 13.However, for clarity in explaining certain fundamental aspects of thepresent invention, FIG. 1 shows only one signpost 11, one tag 12, andone reader 13.

[0024] Focusing first on the signpost 11, the signpost 11 includes amicrocontroller 21. Persons skilled in the art are familiar with thefact that a microcontroller is an integrated circuit which includes amicroprocessor, a read only memory (ROM) containing a computer programand static data for the microprocessor, and a random access memory (RAM)in which the microprocessor can store dynamic data during systemoperation. The signpost 11 also includes a low frequency transmitter 22which is controlled by the microcontroller 21, and which transmits a lowfrequency signpost signal 24 through an antenna 23. The transmitter 22is of a type known to those skilled in the art, and is therefore notillustrated and described here in detail. The antenna 23 of the signpost11 can be a ferrite core and/or planar coil antenna of a known type Theantenna 23 is configured to transmit an omni-directional signal, but itwill be recognized that the antenna could alternatively be configured soas to transmit a signal which is to some extent directional.

[0025] In the embodiment of FIG. 1, the transmitter 22 generates thesignpost signal 24 by effecting amplitude modulation of a carriersignal, which can have a frequency within a range of approximately 30KHz to 30 MHZ. In the embodiment of FIG. 1, and with due regard tocompliance with governmental regulations of various countries regardingelectromagnetic emissions, the carrier frequency is selected to be 132KHz, but could alternatively be some other frequency, such as 132 KHz or13.56 MHZ. A further consideration in the selection of the indicatedfrequency range is that the signpost signals 24 will exhibit near fieldcharacteristics The localized nature of signals in this frequency rangehelps to facilitate compliance with governmental regulations in thespecific context of the present invention, and also helps to minimizereception of these signals by other tags of the type shown at 12, whichare in the general vicinity of the signpost 11 but are beyond anintended transmission range of the signpost signals 24. As known bypersons skilled in the art, a signal with near field characteristics hasa roll-off which is roughly three times higher than the roll-off for asignal with far field characteristics. Consequently, the signpostsignals 24 intentionally have a relatively short transmission range,which in the disclosed embodiment is adjustable but is typically aboutfour to twelve feet. Due to the fact that the signpost signals 24exhibit near field characteristics, the transmission and reception ofthe signpost signals 24 may be viewed as more of a magnetic couplingbetween two antennas, rather than a radio frequency coupling.

[0026] The signpost 11 also includes a power source 26, which wouldtypically be a battery that is capable of powering the signpost forseveral years. However, in situations where the signpost 11 isstationary rather than mobile, it is alternatively possible to power thesignpost 11 from a standard source of 120 VAC power, as indicateddiagrammatically in FIG. 1 by a broken line.

[0027] As shown diagrammatically by a broken line 27 in FIG. 1, themicrocontroller 21 of the signpost 11 can optionally be coupled to thecontrol system 14 by a standard RS-232 serial interface. The RS-232interface would typically be present only where the signpost 11 isfixedly mounted in a stationary location, as opposed to a situationwhere the signpost 11 is mounted on some form of mobile device.Alternatively, the RS-232 interface could couple the signpost 11 to thereader 13, because the reader 13 would typically be closer to thesignpost 11 than the control system 14. In that case, when the controlsystem 14 wished to communicate with the signpost 11, it would do sothrough the reader 13. Although the interface 27 in FIG. 1 is an RS-232interface, it will be recognized that it could alternatively be someother suitable interface, such as an Ethernet interface, an RS-485interface, or a wireless interface.

[0028] The signpost 11 transmits the signpost signal 24 at periodicintervals. The time interval between successive transmissions may beconfigured to be relatively small, such as 100 msec, or relative large,such as 24 hours, depending on the particular circumstances of a givensignpost 11 relative to the rest of the system. Each signpost signal 24transmitted by the signpost 11 includes several different elements ofinformation, which will now be discussed in association with FIG. 2.

[0029] More specifically, FIG. 2 is a diagrammatic view of a digitalword 36 having several different fields of information which arediscussed below. The bits of the digital word 36 are transmitted in thesignpost signal 24 by serially modulating the bits of the word 36 ontothe 132 Kz carrier using amplitude modulation, as mentioned above. Thebits of the words 36 are transmitted serially from left to right in FIG.2. The first field is a preamble 41, which is a predefined pattern ofbits that will allow a device receiving the signal to recognize that thesignpost signal is beginning, and to synchronize itself to the signpostsignal. In the disclosed embodiment, the preamble is approximately 8bits, but the specific number of bits can vary in dependence oncharacteristics of the particular receiver which is expected to be usedto receive the signpost signal.

[0030] The next field 42 in the word 36 is a signpost code, which in thedisclosed embodiment is a 12-bit integer value that uniquely identifiesthe particular signpost 11 which is transmitting the word 36. Asmentioned above, the system 14 may have a number of signposts 11, andthe use of different signpost codes 42 by different signposts permitsthe system to distinguish signpost signals transmitted by one signpostfrom those transmitted by another, in a manner discussed in more detaillater.

[0031] This does not mean that this system could never have twosignposts with exactly the same signpost code. For example, twosignposts might be stationarily mounted in close proximity to each otherand configured to independently transmit effectively identical signpostsignals 24, not in synchronism, in order to increase the likelihood thata receiver will pick up the signpost signal from at least one of the twosignposts. In effect, this represents a level of redundancy, in order toincrease reliability and accuracy. A different possible scenario is thattwo signposts 11, which are fixedly mounted at respective locationsremote from each other, could conceivably use exactly the same signpostcode 42. For example, if they each communicated with the control system14 through a respective different reader 13, the control system 14 wouldhave the capability to distinguish them from each other.

[0032] The next field in the word 36 of FIG. 2 is a tag command 43,which is a command to the beacon tag 12 that can affect the operation ofthe beacon tag 12. The tag command field 43 is a 2-bit field. Since thepurpose of the tag command field 43 is to affect the operation of thebeacon tag 12, a discussion of specific examples of these commands willbe deferred until after the beacon tag 12 has been described in moredetail. The next two fields in the word 36 are a control command 44 anda parameter 45, which are related. In the disclosed embodiment, thecontrol command 44 is a 4-bit field, and a parameter 45 is an 8-bitfield. The control command 44 is similar to the tag command 43, to theextent that they each instruct the tag 12 to do something. Thedifference is that the control commands 44 generally requires anaccompanying parameter 45, whereas the tag commands 43 do not useparameters. A discussion of the control commands 44 is deferred untillater, after the tag 12 has been discussed in more detail.

[0033] The next field in the word 36 is an extension flag 46, which is a1-bit field. In the disclosed embodiment, this field is always a binary“0”! for the word format 36 of FIG. 2. It is provided for the purpose offacilitating future compatibility. For example, if it was necessary atsome future time to modify the format of the word 36, the flag 46 wouldbe set to a binary “1” in each word having the new format, so that adevice receiving the signpost signal 24 could determine whether the word36 received in that signal had the original format shown at 36 in FIG.2, or the new format.

[0034] The next field in word 36 is an error control field 47. Sincecommunications between the signpost 11 and other devices are essentiallyone-way transmissions, and since many applications for the apparatus 10of FIG. 1 involve environments that have relatively high noise levels,it is important for a receiving device to be able to evaluate whetherthe word 36 it received in a signpost signal is correct, or whether ithas errors. Consequently, the error control field 47 is included toprovide a degree of forward error correction (FEC). In the disclosedembodiment, the error control field 47 contains eight parity bits, butthe number of parity bits may be different if the total number of bitsin the word 36 is changed, or if a different one of several well-knownparity schemes is selected for use. In addition to use of the errorcontrol field 47, the overall level of reliability and accuracy can alsobe increased by causing a device which receives the signpost signal 24to save and compare two successive transmissions of a given signpostsignal 24, in order to verify that they are completely identical.

[0035] The last field in the word 36 is a packet end field 48. Thisfield signals to a receiving device that the transmission is ending. Inthe embodiment of FIG. 2, the packet end field 48 has eight bits whichare all set to a binary “0”.

[0036] As mentioned above, the signpost signal 24 is typicallytransmitted in a relatively noisy environment. In order to ensurereliable signal detection, known techniques may be employed to improvethe signal to noise ratio (SNR) In the disclosed embodiment of FIG. 1,the amplitude modulation of the 132 KHz carrier is effected using thewell-known technique of amplitude shift keying (ASK), in order toimprove the SNR. Alternatively, frequency shift keying (FSK) or phaseshift keying (PSK) could be used to achieve an even higher SNR. However,FSK or PSK would typically require additional front-end analog circuitryin each tag 12. Therefore, and since an object of the present inventionis to implement both the signpost 11 and the tag 12 at a low cost, ASKis used in the embodiment of FIG. 1.

[0037] As noted above, communications between the signpost 11 and thebeacon tag 12 are one-way communications involving the signpost signals24. With this in mind, it is desirable to provide a degree of securitythat ensures the beacon tag 12 will react only to valid signpost signals24, especially with respect to the commands in fields 43-45. Therefore,the fields 42-47 in the word 36 can be subjected to security protectionusing well-known encryption and/or password techniques.

[0038] As discussed above, the signpost 11 in the embodiment of FIG. 1transmits the signpost signal 24 at a frequency of 132 Kz, in order toprovide those signals with an effective range which does not exceedabout twelve feet. In some applications, however, there may be a needfor a somewhat longer range for the signpost signals. In that case, thesignpost signals 24 could be transmitted using a different carrier, forexample a high frequency microwave carrier of approximately 2.4 GHz,which would be effective in providing a range of about twenty-five feet.Of course, use of signals at this microwave frequency means that thesignpost 11 should generally have a line-of-sight relationship to eachtag 12 to which it is transmitting.

[0039] Turning to the beacon tag 12, the tag 12 includes a receivingantenna 61 which receives the signpost signals 24 transmitted by thesignpost 11. The antenna 61 is coupled to a low frequency receiver 62 ofa known type, which is designed to receive the signpost signals 24,extract from them the information shown in word 36 of FIG. 2, and thensupply this information to a microcontroller 63 of the tag 12. The tag12 also includes a timer 66 which can be used by the microcontroller 63to measure time intervals that are discussed later. The tag 12 furtherincludes a power source 67, which is typically a battery. However, in asituation where the tag 12 is stationarily mounted, the power source 67could alternatively be an AC/DC adapter which is powered by an externalsource of 120 VAC power, as indicated diagrammatically by a broken linein FIG. 1.

[0040] The microcontroller 63 controls an ultra high frequency (UHF)transmitter 68 of a known type, which in turn is coupled to atransmitting antenna 71 of a known type. In the disclosed embodiment,the antenna 71 is omni-directional, but it will be recognized that theantenna 71 could alternatively be configured to be directional. Usingthe transmitter 68 and the antenna 71, the microcontroller 63 of the tag12 can transmit beacon signals 72 to the reader 13. In the embodiment ofFIG. 1, the beacon signals 72 are generated by FSK modulation of certainbeacon information onto a carrier signal having a frequency of 433.92MHz. A suitable alternative frequency is 915 MHz, but the frequency of433.92 MHz is used in the disclosed embodiment because it is availablefor use in a wider number of countries than 915 MHz under prevailinggovernmental regulations for transmission of electromagnetic signals.The transmission range for the beacon signals 72 is substantially longerthan that for the signpost signals, and in the disclosed embodiment canbe up to about 300 feet. The beacon signals 72 are transmitted using atechnique known in the art as a slotted aloha protocol, to reduceinterference between beacon signals transmitted by different beacontags.

[0041] In the disclosed embodiment, the beacon information transmittedin the beacon signals 72 may take one of two different forms, both ofwhich are shown in FIG. 3. More specifically, if the beacon tag 12 hasreceived a valid signpost signal 24 through the antenna 61 and thereceiver 62, the beacon information transmitted in the beacon signal 72will have the word format shown at 81 in FIG. 3. In contrast, duringperiods of time when the beacon tag 12 is outside the transmission rangeof the signpost signals 24 from any signpost 11, the beacon informationtransmitted in the signal 72 will have the word format shown at 82 inFIG. 3. In the disclosed embodiment, fields 87-88, 91-92 and 97 (andfields 93 and 96 in the case of the word 81) are all transmitted usingManchester encoded FSK modulation at 27.7 Kbps.

[0042] The word format 81 will be discussed first. It begins with apreamble 86, which is functionally comparable to the preamble 41 of theword 36 shown in FIG. 2. In the disclosed embodiment, the preamble 86lasts 1.296 microseconds, and includes 20 cycles which each include a 30microsecond logic high and a 30 microsecond logic low, followed by onecycle which includes a 42 microsecond logic high and then a 54microsecond logic low. The next field in the word 81 is a 1-bit formatfield 87, which is provided to indicate to a receiving device which ofthe two formats 81 and 82 in FIG. 3 is the format used for the instantbeacon signal. Thus, the field 87 is always a “1” bit in word 81, and a“0” bit in word 82. The next field in the word 81 is a 4-bit tag typefield 88, which is a code that provides some information about how theparticular tag 12 is being used in the system. In this regard, the codemay indicate that the tag is stationarily mounted, for example on aceiling, or may indicate that the tag is mounted on some form of mobiledevice. Further, where the tag is mounted on a mobile device, the tagtype code 88 can provide some information about that mobile device, suchas whether that mobile device has a standard height, or has a taller,high profile height.

[0043] The next field in the word 81 is a 3-bit asset type field 91.Where the tag 12 is attached to some type of mobile device, the assettype field 91 can identify the specific type of mobile device to whichthe tag is attached. For example, the field 91 may indicate that theasset is attached to some form of container, to a trailer or dolly onwhich a container can be transported, or to a tractor capable of pullingtrailers having containers thereon.

[0044] The next field in the word 81 is a signpost code 93. This isidentically the signpost code extracted at 42 from the signpost word 36that was most recently received by the beacon tag 12. In the disclosedembodiment, the word 81 has only one signpost code field 93.Consequently, a system according to the disclosed embodiment should beconfigured so that each beacon tag 12 is within the transmission rangeof only one signpost at any given point in time. However, it will berecognized that additional fields could be provided for additionalsignpost codes in the word 81, so that the tag 12 could be within thetransmission range of multiple signposts at the same time, whilereceiving and reporting signpost codes for all of those signposts.

[0045] The next field in word 81 is a last command field 96, which isidentically the last command that was received in either of the fields43 or 44 of the signpost word 36 provided by the signpost having thesignpost code which is present in the field 93. This providesconfirmation to the control system 14 that the tag 12 received thisparticular command from the signpost 11.

[0046] The next field in the word 81 is an error control field 97. Inthe disclosed embodiment, this is a 16-bit field containing a cyclicredundancy code (CRC) of a known type, which is calculated using theinformation in fields 87-88, 91-93 and 96. The beacon signals 72transmitted by the tag 12 to the reader 13 are essentially one-waysignals, and the error control field 97 is therefore provided to givethe reader 13 a degree of capability to detect and correct some errorsin a received word 81. The reader 13 can also increase accuracy andreliability by receiving and comparing two successive beacon signals 72and verifying that they are identical.

[0047] The last field in the word 81 is a packet end field 98, which inthe disclosed embodiment is a logic low of 36 microseconds. The packetend field 98 indicates to a receiving device that the field 98 is theend of the word 81 which is currently being received.

[0048] Turning to the alternative format 82 of the beacon word, thebasic difference from the word 81 is that the fields 93 and 96 of theword 81 are omitted from the word 82. This is because the fields 93 and96 contain information extracted from the last received signpost word36. In contrast, as mentioned above, the beacon word 82 is used insituations where the beacon tag 12 is not currently receiving anysignpost signals, and thus has no current information to put into thefields 93 and 96. Therefore, the fields 93 and 96 are omitted in wordformat 82, In theory, it would be possible to use the word format 81even when the tag 12 is not currently receiving information from anysignpost, and to simply put a “dummy” code such as all zeros into eachof the fields 93 and 96. However, governmental regulations regardingradio transmissions tend to involve a balancing between factors such asthe power level at which a beacon signal 72 is transmitted, the timeinterval between successive transmissions of beacon signals 72, and theamount of information present in each beacon signal. By using the beaconword format 82 when the fields 93 and 96 are not needed, the duration ofthe transmission of the beacon signal 72 is reduced, which in turnfacilitates compliance with governmental regulations.

[0049] There are two other differences between the beacon word format 82and the beacon word format 81. First, the field 87 is always a binary“1” in word 81, and a binary “0” in the word 82, as discussed above.Second, the CRC value used in error control field 97 is calculated usingfields 87-88 and 91-92 in beacon word 82, because the fields 93 and 96are not present, and thus cannot be taken into account.

[0050] Each transmission of the beacon signal 72 is similar to thetransmission of a signpost signal 24, in that it is a short burst at thecarrier frequency which includes one occurrence of either the word 81 orthe word 82 (FIG. 3). The beacon tag 12 uses one technique forsequencing the beacon transmissions 72 when the tag 12 is not currentlyreceiving any valid signpost signals 24, and uses a different techniquefor sequencing the beacon signals 72 in response to the receipt of avalid signpost signal 24.

[0051] In this regard, during any given time interval, a number ofdifferent beacon tags 12 may all be trying to transmit respectivedifferent beacon signals 72 to a given reader 13, and it is inevitablethat two or more of these tags will attempt to transmit beacon signals72 at the same time, such that the signals interfere or “collide” witheach other at the reader 13. The two different techniques used fortransmitting the beacon signals 72 each seek to reduce the likelihoodthat any two tags 12 will transmit beacon signals 72 in a synchronizedmanner that causes successive beacon transmissions 72 from each of thesetwo tags to repeatedly collide. Consequently, each technique is intendedto ensure that, even if two tags each happen to transmit a beacon signal72 at approximately the same point in time, the next successive beaconsignals from these two tags will not occur at the same point in time.

[0052] In more detail, and beginning with the situation where the tag 12is not currently receiving any valid beacon signals 24, the tag 12operates in a normal transmission mode in which it divides ongoing timeinto a succession of time slots having equal lengths, for example 60second time slots, and in which it effects transmission of one beaconsignal 72 within each time slot, at a randomly selected time within thattime slot. In the disclosed embodiment, the random selection is actuallydone with a pseudo-random calculation of a known type, which closelyapproximates a truly random determination. References herein to randomdeterminations are intended to include techniques such as pseudo-randomdeterminations.

[0053] When the tag 12 receives a valid signpost signal 24, itimmediately interrupts the normal mode of transmission and switches to aspecial mode of transmission. At the end of the special mode oftransmission, it reverts back to the normal mode. The special mode isdiscussed in association with FIG. 4, in which the horizontal axis atthe bottom represents the progression of time from left to right. Thevertical line at the left side of FIG. 4 represents the point in time atwhich a valid signpost signal is received, and represents the point intime at which the tag 12 responds by switching from the normal mode tothe special mode. The special mode involves five successive timeintervals 111-115, which are each discussed separately below. After thelast time interval 115 of the special mode, the tag 12 reverts from thespecial mode to the normal mode, where operation in the normal mode isrepresented by the time interval 116.

[0054] Time interval 111 involves N1 successive time slots which eachhave a duration of T1. In the disclosed embodiment, N1 is 5, and T1 is0.1 seconds. The tag 12 transmits the beacon signal 22 once during eachof these five time slots, at a randomly selected point within that timeslot. These five time slots are represented diagrammatically in FIG. 4by the spaces between the short vertical lines within time interval 111along the horizontal axis at the bottom of FIG. 4.

[0055] It will be noted that the operation of the tag during interval111 is somewhat similar to the operation of the tag during its normalmode, but there are two basic differences. First, the time slots in thenormal mode are each about 600 times longer than the time slots in timeinterval 111, and thus the beacon signal 72 is being transmitted anaverage of 600 times more often than in the normal mode.

[0056] Second, during the time interval 111, the tag 12 transmits eachbeacon signal 72 at a power level P1, which is 24 dB lower than a powerlevel P2 used during normal operation. As mentioned above, governmentalregulation of UHF transmissions can involve a degree of balancingbetween the duration of each transmission, the time interval betweensuccessive transmissions, and the power level of the transmissions.Consequently, since the transmissions in time interval 111 have a longerduration than transmissions in the normal mode (because they involvebeacon word 81 of FIG. 3 rather than beacon word 82), and since they aresent an average of 600 times as often, the reduced power level P1 isused for these transmissions in order to facilitate compliance withgovernment regulations. The power level which is being used at any givenpoint in time is set forth along the top of FIG. 4.

[0057] Time interval 111 is followed by time interval 112, which is adelay or wait state having a duration T5, where T5 is 1 second in thedisclosed embodiment. During the time interval 112, the tag 12 does nottransmit any beacon signals 72.

[0058] Time interval 112 is followed by time interval 113, which ishandled in a manner similar to time interval 111, except that someparameters are different. In particular, time interval 113 includes N2successive time slots which each have a duration of T2. In the disclosedembodiment, N2 is 3, and P2 is 1 second. A single beacon signal 72 istransmitted during each T2 time slot, at a randomly-selected time withinthat time slot. Beacon signals 72 that are transmitted during the timeinterval 113 are transmitted at the reduced power level P1 which wasused in time interval 111.

[0059] Time interval 113 is followed by time interval 114, which is adelay or wait state similar to time interval 112. In particular, nobeacon signals 72 are transmitted, and the time interval has a durationof T6, which in the disclosed embodiment is 10 seconds.

[0060] Time interval 114 is followed by the time interval 115, whichinvolves activity similar to the time intervals 111 and 113. Inparticular, time interval 115 includes N3 time slots which each have aduration of T3. In the disclosed embodiment, N3 is 3, and P3 is 10seconds. A single beacon signal 72 is transmitted during each of thesetime slots, at a randomly-selected point within the time slot. In thetime interval 115, the tag 12 reverts to the higher power level of P2.In this regard, it will be noted that the average rate of transmissionof beacon signals in time interval 115 is about one-tenth of the averagerate of transmission of beacon signals in time interval 113, and isabout one one-hundredth of the average rate of transmission in timeinterval 111. Thus, and with reference to the above-discussed balancingbetween the duration of transmissions, the time interval betweentransmissions, and the power level, the tag 12 can revert to the higherpower level P2 as a result of the significant decrease in the averagerate of transmissions, while still complying with governmentregulations.

[0061] Time interval 115 is followed by time interval 116 which, asmentioned above, represents a reversion to the normal mode of operation.In particular, the tag 12 continuously divides ongoing time intosuccessive time slots that each have a duration T4, where T4 is 60seconds. These beacon signals are each transmitted at the higher powerlevel P2, using the shorter format of the beacon word which is shown at82 in FIG. 3, The time interval 116 does not have a specified duration,and will continue until the tag 12 receives a further valid signpostsignal which causes it to again switch to the special mode and carry outthe beacon sequence shown in FIG. 4.

[0062] The foregoing discussion mentions various parameters, includingN1-N3, T1-T6, and P1-P2, and gives specific values for some of theseparameters. The specific values given for these parameters are thoseused in the disclosed embodiment, but it is within the scope of thepresent invention to vary these parameters.

[0063]FIG. 5 is a flowchart showing in a different form the beaconsequence discussed above in association with FIG. 4. In FIG. 5, themicrocontroller 63 of the beacon tag 12 enters block 131 in response toreceipt of a valid signpost signal 24. Block 131 corresponds to timeinterval 111 in FIG. 4. In block 131, the beacon tag transmits a beaconsignal with the power level P1 at a random time within each of N1successive time slots that each have a duration T1.

[0064] The system then progresses to block 132 in FIG. 5 whichcorresponds to time interval 112 in FIG. 4, In particular, the beacontag waits for a time interval T5, without transmitting any beaconsignals. The system then progresses to block 133, which corresponds totime interval 113 in FIG. 4. In block 113, the beacon tag transmits abeacon signal with power level P1 at a random time within each of N2successive time slots that each have a duration T2.

[0065] The system then progresses to block 134, which corresponds totime interval 114. In block 134, the system waits for a time interval T6without transmitting any beacon signals, and then progresses to block135. Block 135 corresponds to time interval 115 in FIG. 4. In block 135,the system transmits a beacon signal with power level P2 at a randomtime within each of N3 successive time slots that each have a durationP3.

[0066] From block 135, the system progresses to block 136, whichcorresponds to time interval 116 in FIG. 4. The system stays in block136 indefinitely, until a further valid signpost signal is received.While in block 136, the beacon tag transmits a beacon signal with thepower level P2 at a random time within each of a series of successivetime slots that each have a duration of T4. If a further valid signpostsignal is received, then the beacon tag immediately interrupts itsactivity in block 136 and returns to block 131, as indicateddiagrammatically by the broken line 137, in order to again carry out thebeacon sequence which is represented by blocks 131-135.

[0067]FIG. 6 is a high-level flowchart depicting the operation of thebeacon tag 12. With reference to FIG. 1, the beacon tag 12 has a reducedpower mode in which the transmitter 68 is off, the timer 66 is active,the receiver 62 is active, and the microcontroller 63 is in a reducedpower or “sleep” mode, from which it can be awakened by either thereceiver 62 or expiration of the timer 66. The flowchart of FIG. 6begins at a point in time when the beacon tag 12 wakes up from thereduced power mode, either because the receiver 62 has received asignpost signal, or because the timer 66 has expired.

[0068] The microcontroller 63 of the tag 12 proceeds from block 151 toblock 152, where it checks to see if the timer 66 has just expired. Ifnot, then it knows that the receiver 62 has received a signpost signal,and it proceeds to block 153, where it extracts and stores the signpostcode (42 in FIG. 2) from the received signpost signal. Then, controlproceeds to block 156, where the beacon tag checks to see whether thereceived signpost signal also includes a command in either of fields 43and 44 (FIG. 2). If so, then the tag proceeds to block 157, where itexecutes the command. Then the tag proceeds to block 158, where itreturns to its reduced power “sleep” mode.

[0069] Looking again at block 156, if the beacon tag were to determinethat the signpost signal did not include a command, then the beacon tagwould have proceeded to block 161, where it resets the beacon sequence.This corresponds to the broken line 137 in FIG. 5, where the tag leavesthe normal mode of operation represented by block 136, and returns toblock 131 in order to carry out the special beacon sequence which isrepresented by blocks 131-135 in FIG. 5 and by time intervals 111-115 inFIG. 4.

[0070] Then, at block 162, the beacon tag determines the next point intime at which it needs to transmit its beacon signal according to thebeacon sequence. Since the beacon sequence has just been restarted inblock 161, this will be a determination of the point in time to transmitthe beacon signal within the first time slot of the time interval 111 inFIG. 4. As discussed above, this will involve a random determination ofa point in time within the time slot, for example using a pseudo-randomtechnique of a known type. Once this point in time has been selected,the beacon tag 12 sets the timer 66 (FIG. 1) in block 163 of FIG. 6, sothat the timer will expire at the proper point in time to allowtransmission of the next beacon signal, and then the beacon tag 12returns to the sleep mode at block 158.

[0071] Returning to block 152 in FIG. 6, if it had been determined thatthe microcontroller 63 was awakened from the sleep mode because thetimer 66 expired, the microcontroller 63 would have proceeded from block152 to block 167. In block 167, a determination is made of whether thetimer expired because it is time to transmit the next beacon signal. Ifnot, then the beacon tag proceeds directly to block 158, where itreturns to the sleep mode. Otherwise, it proceeds from block 167 to 168,where it effects transmission of its beacon signal 72 (FIG. 1) It thenproceeds to block 162, where it picks the transmit time for its nextsuccessive beacon signal. Then, at block 163, it sets the timer toexpire at the point in time that it determined. Then, at block 158, itreturns to the reduced power sleep mode.

[0072] At an earlier point in this discussion, in association with thediscussion of FIG. 2, it was indicated that the command fields 43-45would be described in due course. The following is a discussion of thosefields.

[0073] The tag command field 43 is a 2-bit field which can be used toinstruct a beacon tag 12 (1) to turn itself off (which is actually a lowpower sleep mode in which no beacon signals are transmitted), (2) toturn itself on (which is a mode in which beacon signals are transmittedin the manner described above in association with FIGS. 4-6), (3) tooperate at a fast beacon rate, or (4) to operate at a slow beacon rate(where the slow rate uses a duration for each time slot T4 of FIG. 4that is longer than the duration used for the fast rate).

[0074] Turning to the control command field 44 and the parameter field45, it was mentioned above that the parameter field 45 contains aparameter needed to implement a command specified by the control commandfield 44. One command which can be specified in the control commandfield 44 is an instruction to the beacon tag 12 to set the beacon codethat it puts into field 92 (FIG. 3), and in that case the parameterfield 45 would contain the new beacon code. Another command which can bespecified by the control command field 44 is an instruction to thebeacon tag 12 to set a password or an encryption key used for security,as discussed above, and the parameter field 45 would contain the newpassword or encryption key. Yet another command which can be specifiedby the control command field 44 is an instruction to the beacon tag 12to set the tag type code that it puts into field 88 (FIG. 3), or theasset type code that it puts into field 91, and the parameter field 45would contain the new tag type code or asset type code. Still othercommands in the control command field 44 could instruct the beacon tagto change any one of the various parameters discussed above inassociation with FIGS. 4 and 5, including P1, P2, N1, N2, N3, T1, T2,T3, T4, T5, and T6, and the parameter field 45 would contain the newvalue for the specified parameter. It will be recognized that there arestill other commands which could be sent to the tag 12 using the controlcommand field 44 and, where needed, the parameter field 45.

[0075] Referring again to FIG. 1, the reader 13 will now be described ingreater detail. The reader 13 includes two antennas 211 and 212 whichare of a known type, and which are each suitable for receiving UHFwireless signals. The reader 13 also includes two UHF receivers 213 and214, which each have an input coupled to a respective one of theantennas 211 and 212. The reason that the reader 13 has two UHF antennas211-212 and two UHF receivers 213-214 is that the antennas 211-212 arearranged to extend perpendicular to each other. The reader 13 is capableof determining which of the two antennas 211-212 is producing thestrongest output in response to a given beacon signal 72. The reader 13then selects the stronger output for use as the received version of thatparticular beacon signal.

[0076] The reader 13 also includes a decoder 217 of a known type, whichhas two inputs that are each coupled to an output of a respective one ofthe receivers 213-214. The decoder 217 processes the signals received byeach of the receivers 213-214, in order to extract usable informationtherefrom, which can then be passed to a microcontroller 221 of thereader 13. A real time clock (RTC) circuit 222 is coupled to themicrocontroller 221. Further, the reader 13 includes a network interface223. A network 226 is of a type known in the industry as an Ethernetnetwork, and couples the network interface 223 of the reader 13 to thecontrol system 14, in order to facilitate communication between thereader 13 and the control system 14 The basic function of the reader 13is to receive beacon signals 72 from various beacon tags (such as thetag 12), verify that each received beacon signal is valid, perform errordetection and correction where needed, extract information such as oneor more of the fields shown at 87-88, 91-93 and 96 in FIG. 3, and thenpass this extracted information on to the control system 14.

[0077]FIG. 7 is a diagrammatic top view of a system 240 which representsone practical application of an apparatus of the type shown at 10 inFIG. 1. The system 240 of FIG. 7 includes a plurality of signposts,sixteen of which are shown at 241-256 in FIG. 7. Each of the signposts241-256 is identical to the signpost shown at 11 in FIG. 1, except thatthey each use a respective unique signpost code 42 (FIG. 2). Thesignposts 241-256 have been given different reference numerals in FIG. 7in order to facilitate a discussion of how the system 240 operates.

[0078] The signposts 241-256 are each stationarily mounted, for exampleon the ceiling of a warehouse or other industrial facility. The sixteensignposts 241-256 are arranged in a regular 4×4 array. The broken linecircle which extends around each signpost in FIG. 7 is a diagrammaticrepresentation of the effective outer limit of the transmission range ofthe signpost signals emitted by that signpost. As discussed above, eachsignpost has a limited transmission range of only about 12 feet or less,and the spacing between the signposts 241-256 has thus beenintentionally selected so that no two signposts have overlappingtransmission ranges. Although sixteen signposts 241-256 are shown inFIG. 7, this 4×4 array is just a portion of a much larger array thatcovers a much larger area. However, the array shown in FIG. 7 issufficient for purposes of explaining certain principles of the presentinvention.

[0079] A reader 261 is stationarily mounted within the array ofsignposts 241-256, for example on the same ceiling that supports thesignposts. The reader 261 is identical to the reader shown at 13 in FIG.1, but is given a separate reference numeral here for clarity The system240 would actually include a number of other equivalent readers atspaced locations, but only one reader 261 is illustrated in FIG. 7 inorder to facilitate a clear explanation of certain features of theinvention.

[0080] Five beacon tags 271-275 are also depicted in FIG. 7. The beacontags 271-275 are each effectively identical to the beacon tag shown at12 in FIG. 1, but have been given separate reference numerals forclarity in the discussion which follows. For purposes of the followingexplanation, it is assumed that the beacon tags 271-275 are each mountedon a different mobile device, such as a container, a pallet, a forklift,a trailer which can support a container, a tractor which can pull atrailer, or some other type of mobile device.

[0081] Focusing first on the beacon tag 271, it will be noted from FIG.7 that this tag is currently within the transmission range of thesignpost 241. Consequently, the beacon tag 271 will be receivingsignpost signals 281 from the signpost 241, and will be transmittingbeacon signals 282 to the reader 261. The beacon signals 282 willinclude the beacon code unique to the beacon tag 271, as well as thesignpost code unique to the signpost 241. Consequently, since thissignpost code and this beacon code are received in combination with eachother in the beacon signal 282, the control system associated with thereader 261 can determine that the beacon tag 271 is presently within thetransmission range of the signpost 241. This in turn means that themobile device which carries the beacon tag 271 is currently very closeto the signpost 241. Since the control system knows the physicallocation of the signpost 241, the system can make a relatively accuratedetermination of the current location of the mobile device which carriesthe beacon tag 271, localized to the transmission range of the signpost241. In particular, the system can determine the current location of thebeacon tag 271 and its associated mobile device to an accuracy of about12 feet, which is the radius of the transmission range of the signpost241. It will be recognized that this capability is due in part to thefact that the signpost signals have a relatively local transmissionrange, whereas the beacon signals have a transmission range which isabout 30 times farther than the transmission range of the signpostsignals.

[0082] For purposes of comparison, assume for a moment that thesignposts 241-256 were all omitted from the system 240 of FIG. 7. Inthat case, the beacon signals 282 from the beacon tag 271 would eachinclude the unique beacon code of the tag 271, but would not include anysignpost code. By analyzing the strength of the beacon signal 282, asreceived at the reader 261, the control system associated with thereader 261 could make a very rough estimate of the distance between thetag 271 and reader 261. However, it would be difficult for the controlsystem to accurately determine which direction the beacon signal 282came from. In this regard, even though the reader 261 has two orthogonalantennas (equivalent to those shown at 311-312 in FIG. 1), the reader261 would not know whether the beacon signal 282 arrived from onedirection, or from a diametrically opposite direction.

[0083] Still assuming that no signposts are present in the system, butthat a second reader is provided in a manner so that both readersreceive the beacon signals 282, the control system could estimate thedistances from the beacon tag 271 to each of the two readers. With thisinformation, it would be possible to carry out a standard triangulationcalculation in order to attempt to estimate the location of the beacontag 271. But due to rather wide tolerances in the ability to estimatedistances from the beacon tag to each reader based on beacon signalstrength, even triangulation produces only a very coarse estimate oflocation, which is not particularly accurate and reliable. It will thusbe recognized that, through use of the signposts 241-256 in FIG. 7, asignificantly more accurate and reliable determination can be made ofthe current location of the beacon tag 271.

[0084] In FIG. 7, the mobile device associated with the beacon tag 275is currently in a location where the beacon tag 275 is not within thetransmission range of any of the signposts 241-256. Thus, the reader 261is receiving a beacon signal from the beacon tag 275, but the beaconsignal includes only the beacon code of the tag 275, and does notinclude a signpost code from any of the signposts 241-256. Therefore,the tag 275 is temporarily situated where the system cannot determineits location as accurately as if it were currently within thetransmission range of any of the signposts. Nevertheless, the system 240may still have a relatively accurate idea of the current location of thetag 275, by tracking it over time.

[0085] For example, the system may know that the tag 275 reached itscurrent location by moving through the transmission range of signpost243 and then through the transmission range of signpost 242, and thesystem may thus predict that the tag 275 will soon enter thetransmission range of signpost 245. Therefore, even though tag 275 isnot currently within the transmission range of any signpost, the systemstill has a better idea of the current location of the tag 275 thanwould be the case if there were no signposts at all. A furtherconsideration in this regard is that, within a warehouse or otherindustrial facility, there are often defined paths that mobile devicestend to follow through the facility. Accordingly, the system may be wellaware that there is a defined path which extends successively pastsignpost 243, signpost 242, and signpost 245. This will provide thesystem with an even better ability to accurately estimate the currentlocation of tag 275, even when it is not currently within thetransmission range of any of the signposts 251-256.

[0086] It is possible for two or more beacon tags to be simultaneouslywithin the transmission range of a single signpost, such that all ofthose beacon tags are simultaneously receiving the same signpost signalemitted by that signpost. This is the case with beacon tags 272-274 inFIG. 7, which are all within the transmission range of the signpost 248.The reader 261 receives a separate beacon signal from each of the tags271-274, and each of these beacon signals includes the unique beaconcode of the corresponding beacon tag, in combination with the signpostcode of the signpost 248. Thus, the control system associated withreader 261 can distinguish the beacon tags 272-274 from each other, dueto their unique beacon codes, and can also determine that all of thesebeacon tags are currently at locations within the transmission range ofthe signpost 248.

[0087] Although FIG. 7 shows an array of signposts 241-256 which arestationary, and several beacon tags 271-275 which are mobile, thestationary and mobile characteristics of the signposts and beacon tagscan be reversed. In this regard, FIG. 8 is a diagrammatic top view of asystem 300 which has sixteen stationary beacon tags 301-316, each ofwhich is equivalent to the beacon tag 12 of FIG. 1. These beacon tagsare arranged in a 4×4 array, with spacing equivalent to that used forthe signposts 241-256 in FIG. 7. A reader 319 is provided at a centrallocation within the array, and is also stationary. A signpost 322 ismounted on a mobile device, which can move within the facility, and thuscan move with respect to the stationary beacon tags 301-316. At thepoint in time depicted in FIG. 8, the mobile device carrying signpost322 is at a location near the beacon tag 301, such that the beacon tag301 is within the transmission range of the signpost 322.

[0088] The signpost 322 is transmitting a signpost signal, but the onlybeacon tag which can currently receive that signal is the beacon tag301. Thus, the beacon tags 301-316 are each transmitting a respectivebeacon signal to the reader 319, and each of these beacon signalsincludes a unique beacon code, but only the beacon signal from the tag301 also includes the unique signpost code that it is receiving in thesignpost signal from the signpost 322. The control system associatedwith the reader 319 will know the physical location of each of thestationary beacon tags 301-316. Thus, the location of the mobile deviceassociated with the signpost 322 can be determined with the same degreeof accuracy achieved in the system of FIG. 7, because the control systemfor the embodiment of FIG. 8 knows that the distance between thesignpost 322 and the beacon tag 301 must be less than the radius oftransmission of the signpost signals from signpost 322, or in otherwords approximately 12 feet. If the signpost 322 moves until it is closeto the beacon tag 302, then the beacon tag 301 will no longer be withinthe transmission range of the signpost signals from signpost 322, butthe beacon tag 302 will be within that transmission range. Consequently,the beacon tag 301 will stop transmitting the signpost code fromsignpost 322 in its beacon signal, and the beacon tag 302 will starttransmitting this signpost code in its beacon signal. As a result, thecontrol system associated with reader 19 can track the movement of themobile device associated with signpost 322.

[0089] One difference between the systems of FIGS. 7 and 8 is that,since the beacon signal from any beacon tag is configured to includeonly one signpost code, each beacon tag should never be within thetransmission range of more than one signpost at any given point in time.In the system of FIG. 7, this is assured by the stationary mounting ofthe signposts 241-256, with appropriate spacing provided between them.In contrast, since the signposts can move in the system of FIG. 8, caremust be taken to ensure that two or more signposts do not come intoproximity with the same beacon tag at the same point in time. This isnot to suggest that the approach of FIG. 7 is more advantageous than theapproach of FIG. 8. One of these approaches may be better for someapplications, and the other may be better for other applications. Infact, it should be evident from the discussion which follows that, insome applications, it would be possible to use a combination of the twoapproaches.

[0090] Certain additional aspects of the present invention will bediscussed below. It is believed that these additional aspects will bemore clearly understood if presented in the context of an example of aspecific application. Therefore, the discussion which follows will focuson a private company which is in the business of overnight packagedelivery. As is well known, companies of this type provide a service inwhich they pick up a package from a sender on one day, and then deliverit to a recipient on the following day, typically before noon. Thesender may be in one city, such as Boston, and the recipient may be adifferent city, such as Tucson.

[0091] On the day that a package is picked up in Boston, the companywill also typically pick up a number of other packages in Boston, whichwill be going to a variety of other cities throughout the country. Thefollowing day, the company will have a number of packages to deliver inTucson, which were picked up the preceding day in a number of differentcities across the country. In order to efficiently handle the routing ofall these packages, existing companies typically provide some form ofhub facility at a major airport. During the night, a container willarrive from a city such as Boston, containing a number of packages thatneed to be delivered in many different cities. The container will beunloaded at the hub facility, and then the packages will be sorted, inorder to group the sorted packages by destination city. Thus, forexample as to Tucson, the sorting process will yield a group of packagesdestined for delivery in Tucson, which arrived at the hub facility in avariety of different containers from a variety of different cities oforigin. The group of packages destined for Tucson will be packed into acontainer, and that container will be transported to Tucson, where thepackages will be delivered locally.

[0092] With respect to a hub facility of the type discussed above, themajority of containers will typically arrive in one of two differentways. First, containers from cities that are not too far from the hubfacility will typically arrive by highway, in various types of trucks.These trucks are commonly referred to as feeders. The containers frommore remote cities will typically arrive by airplane. The shapes andsizes of the containers which arrive by airplane and by truck can varywidely. FIG. 9 is a diagrammatic perspective view of one type ofcontainer 381 which is particularly suitable for use in airplanes,because it has a shape which facilitates packing of a number of suchcontainers into the somewhat rounded shape of an airplane body.

[0093] The container 381 of FIG. 9 has an approximately square bottomwall 382, and a top wall defined by a horizontal central portion 383,and two angled portions which extend downwardly at an incline fromopposite sides of the portion 383, one of the angled portions beingvisible at 384. The container 381 has four side walls which each extendvertically upwardly from an edge of the bottom wall to an edge of thetop wall, and two of these side walls are visible at 387 and 388 in FIG.9. The container 381 also has two doors 391 and 392, which can eachpivot between an open position and a closed position. A not-illustratedlatch is provided for securing the doors 391-392 in a closed position,and is contigured in a known manner to permit the doors to be locked orsealed in their closed positions, so that packages cannot be removed byunauthorized individuals as the containers are being transported to orfrom the hub facility.

[0094] The container 381 is itself a known device. According to theinvention, three beacon tags 395-397 are fixedly secured to thecontainer 381 at spaced locations thereon. Each of the tags 395-397 isequivalent to the tag 12 of FIG. 1. The tag 395 is provided on thecentral portion 383 of the top wall of the container. The tags 396 and397 are provided on respective opposite sidewalls of the container 381,closely adjacent diagonally opposite corners of the bottom wall 382, Thevarious types of containers which travel to and from the hub facility bytruck and plane can each be referred to as a unit load device (ULD). Thecontainer 381 of FIG. 9 is one example of a ULD.

[0095]FIG. 10 is a diagrammatic top view of an installation 400 whichincludes a hub facility 401 of the type discussed above, In the hubfacility 401, packages being transported by an overnight deliveryservice are received from many cities of origin, unpacked, sorted,repacked, and then transmitted to many destination cities. That is, thehub facility 401 in FIG. 10 is essentially a building where packages areunloaded from containers, sorted, and then reloaded into othercontainers. The overall installation 400 includes an inbound section 403and an outbound section 404, which are both external to the physicalbuilding of the hub facility 401. The inbound section 403 relates toreceipt and initial processing of incoming containers, and the outboundsection 404 deals with the processing of outgoing containers.

[0096] A tracking system of the general type discussed above inassociation with FIG. 7 is used for the installation 400, but forclarity is not shown in FIG. 10. This tracking system includes aplurality of spaced signposts mounted on the ceiling of the hub facility401, and at selected other locations throughout the installation 400, asdiscussed below. Further, a plurality of readers are provided throughoutthe installation 400. In the hub facility 401, the readers are mountedon the ceiling. In the inbound and outbound sections, there are readersmounted at entrance and exit gates, on or near unloading equipment, onlight poles, on buildings, on fences, on special supports, or on othersuitable structure which may be present. In general, the signposts areprovided in areas where very accurate estimates of tag location areneeded, using techniques of the type discussed above in association withFIG. 7. In contrast, in areas where a coarser estimate of tag locationis sufficient, signposts can be omitted so that beacon signals do notinclude signpost codes, and estimates of location can be based on thestrength of beacon signals as received at the readers.

[0097] Turning in more detail to the flow of materials through theinstallation 400, an arriving airplane taxis to the inbound section 403,where it is parked at 411. The airplane may be parked at one of twodifferent types of locations. One is commonly referred to as an “onwing” location. This means that the aircraft is parked closely adjacenta building, which typically has a built-in loader or unloader that canbe extended to a door of the plane in order to facilitate loading andunloading. The other type of location is known as an “on ramp” location.This means that the airplane is parked on the tarmac at a locationspaced from any building. Loading and unloading of such a plane arecarried out using know types of mobile loaders and unloaders that cantravel out to the airplane and then back to a building.

[0098] It is a governmental requirement that most electronic deviceswhich are traveling on airplanes must be disabled during the flight, sothat they do not produce any type of wireless electromagnetic signalwhich might interfere with the operation of the plane. Thus, to theextent that any signpost or beacon tags of the type shown at 11-12 inFIG. 1 are traveling by airplane, they must be turned off during theflight, or at least must be in an operational mode where they do nottransmit electromagnetic signals. As discussed above, beacon tags395-397 are provided on ULDs of the type shown at 381 in FIG. 9.Consequently, when these ULDs are unloaded from an airplane, the beacontags need to be turned on. As discussed above, the tag command field 43(FIG. 2) of a signpost signal can turn a beacon tag on or off.Consequently, stationary signposts can provided on or near eachunloading device, or in the region of the airplane unloading operation,in order to turn on all of the beacon tags which are present on the ULDsthat are being unloaded. Alternatively, a handheld signpost could bemanually used by an operator to turn on all of the beacon tags which areon the equipment being unloaded. The beacon tags on the unloaded ULDsthus begin transmitting their beacon signals.

[0099] As noted above, the inbound section 403 has a plurality ofreaders of the type shown at 13 in FIG. 1, at appropriately selectedlocations throughout the inbound section 403. These readers be providedon or near the airplane unloading equipment, on light poles, onbuildings, on fences, on special supports, or on other structure. Thebeacon signals generated by the tags on each ULD will be received by oneor more of these readers, which each will forward the receivedinformation to a central control system of the type shown at 14 inFIG. 1. Since the control system knows which beacon tags are mounted onwhich ULDs, the control system can determine which ULDs have arrived byairplane. The control system can then begin planning how to route eachULD through the installation 400.

[0100] In this regard, there are occasional situations in which a ULDcomes from an origin city which has so many packages going to a singledestination city that all of these packages have been packed into asingle ULD. In that case, the control system can arrange for the ULD tobe transferred directly from the inbound section 403 to the outboundsection 404, because there is no need to do any unpacking, sorting orrepacking. However, the vast majority of ULDs will need to be unpackedand sorted, and thus will need to be routed to the hub facility 401.

[0101] As to all arriving ULDs, the control system will haveelectronically received from each origin city an identification of theULDs being sent, and a list of the specific packages in each such ULD.Thus, depending on the departure schedules for planes traveling todestination cities, the control system can prioritize the order ofhandling arriving ULDs, so that the ULDs containing packages that needto be on the earliest departing flights can be handled before ULDs whichdo not contain packages that need to be on the earliest departingflights. Based on the electronic information received from origincities, the control system knows which ULDs should be on each arrivingplane, and can determine whether one of the expected ULDs is missing, orwhether an extra and unexpected ULD is present. The arrival time of eachULD can also be recorded.

[0102] When the plane is parked on ramp, ULDs can be transported to thehub facility using a train which includes several releasably coupledtrailers or “dollies”, and a tractor or tug which can pull the trailers.Each ULD can be transferred to a respective trailer of the train. Atrain of this type is described in more detail later. In FIG. 10, block142 reflects this transfer of ULDs onto trailers. The train thentransports the ULDs to the hub facility 401. In contrast, if the planeis parked on wing, the ULDs may or may not be transferred to a train ofthis type. They may instead be transported by conveyor, by a device suchas a cart which can be manually pushed, or by some other transportapparatus. Block 413 in FIG. 10 represents the transfer of ULDs from theplane to some form of appropriate device that will facilitate transportof the ULDs.

[0103] At block 416 in 403, each arriving ULD is manually checkedagainst the manifest for the arriving flight. Then, at block 417, theULDs destined for the hub facility are moved to the hub facility. Asmentioned above, readers are provided at selected locations throughoutthe installation 400, including the inbound section 403, the hubfacility 401, and the outbound section 404. Further, signposts of thetype shown at 11 in FIG. 1 are provided at a variety of selectedlocations throughout the installation 400, especially at locations whichthe ULDs must travel past as they are routed through the installation400. Thus, for example, signposts are provided along typical paths oftravel, at doorways, and at various stations where ULDs can temporarilywait for attention, which are referred to as “staging” areas. Using thebasic approach discussed above in association with FIG. 7, the controlsystem can accurately track each ULD throughout the entire installation400.

[0104] The ULDs from the staging area 418 are each eventuallytransported to one of several unloading stations 421. At each unloadingstation, an operator o pens the ULD, and also presses a push button onan adjacent control panel, in order to indicate to the control systemthat the unloading process has started. The operator then unloads all ofthe packages from the ULD, by placing them on conveyors which carry themto a package sort section 422. When the operator finishes unloading aULD, the operator presses a further button on the control panel, toindicate to the control system that the manual unloading process hasbeen completed. In the disclosed embodiment, the control panel at eachunloading station is a physical part of the unloading station. However,it can alternatively be provided in the form of a portable wirelessdevice carried by the operator. The empty ULDs are each taken to astaging area 426, and are eventually moved to a staging area 427, eitherdirectly or through a further staging area 428, which is outside thephysical building of the hub facility 401.

[0105] Referring again to the inbound section 403, and as discussedabove, packages can arrive not only by airplane, but also by truck. Asnoted above, the trucks are referred to as feeders. The feeders cancontain ULDs, in which case the ULDs can be unloaded and handled in amanner very similar to that discussed above in association with anarriving airplane which is parked on ramp. More typically, however, thefeeders include packages which are not packed in ULDS. In that case, thefeeder itself is treated as the container for the packages, and thelower portion of FIG. 10 addresses how this type of feeder is handled.

[0106] In particular, at block 436 the feeder is checked in at the gateof the inbound section 403. A temporary beacon tag similar to that shownat 12 in FIG. 1 is attached to the feeder, for example using somespecial mounting bracket. At the same time, the person attending thegate makes an entry in a computer, which advises the central controlsystem of the arrival of the feeder, and also advises the control systemof the particular beacon tag which has been attached to that feeder, inorder to permit the control system to associate the electronic manifestfor that feeder with the actual physical feeder as it moves through theinstallation 400.

[0107] If the feeder is a truck in the form of a cab pulling a trailer,commonly known as a tractor-trailer combination, the trailer may beseparated from the cab and moved through the installation 400 usingsmall local tractors of a type commonly referred to as yardbirds. On theother hand, if the cab is an integral part of the feeder, the entiretrack may move through the installation 400.

[0108] In any event, at block 437 the feeder is moved from the inboundsection 403 to a staging area 441 that is adjacent to but outside of thebuilding that serves as the hub facility 401. The control systemschedules these feeders for movement to feeder unloading stations, oneof which is shown at 442. Each feeder is unloaded, in a manner similarto that described above for the ULD unloading stations 421. The packagesremoved from the feeders travel to the package sort section 422, forexample by conveyor, while the empty feeders are routed to an emptyfeeder staging area 443.

[0109] In the package sort section 422, all packages that are intendedfor a given destination city are routed to a selected one of severalloading stations 451. An empty ULD is taken from the staging area 427,and is loaded with packages headed for that destination city, eitheruntil the ULD is full or until it contains all of the packages bound forthat destination city. Then, that ULD is transferred to a ULD weighscale section 452, where each ULD is weighed. The weigh scale 452 iscoupled to the central control system, so that the control system willknow the weight of each loaded ULD, and thus can carry out appropriateplanning with respect to how much total weight is being loaded on eachdeparting airplane.

[0110] After each ULD has been weighed at 452, it is moved to anoutbound staging area 453. From there, it is in due course moved out ofthe building through a door having a signpost nearby, and the controlsystem is notified of its exit from the hub facility 401 by virtue ofbeacon signals which are from a tag on the ULD and which include thesignpost code of the signpost. Then, as represented diagrammatically byblocks 456, 457 and 458, these ULDs are transported by trains of thetype discussed earlier to the outbound section 404, where each is loadedon an airplane traveling to the destination city for all of the packageswithin that ULD.

[0111] As mentioned above, government regulations prohibit devices suchas beacon tags from emitting wireless electromagnetic signals duringairplane flight. Accordingly, as each ULD is loaded on a plane, all ofthe beacon tags associated with it are turned off, or at least placedinto a mode in which they do not emit any beacon signals. This can beeffected using a stationary signpost in the region of the loader for theairplane, or using some form of portable signpost operated by a personinvolved with the loading process. As noted above, one of the commandswhich can be present in the tag command field 43 (FIG. 2) of a signpostsignal is a command which turns off any beacon tag that receives thesignal. When an airplane has been loaded with all of the ULDs it isscheduled to carry, the airplane taxis out of the outbound section 404,and then takes off for its destination city.

[0112] Some of the packages sorted in the sort section 422 are scheduledto depart by truck rather than airplane, for example where they are tobe delivered to destinations that are not far from the installation 400.The sorting process routes these packages to feeder load stations, oneof which is shown at 461. An empty feeder from the feeder staging area443 is moved to one of the feeder load stations 461, where it is loadedwith sorted packages that it is carry to one or more relatively localdelivery centers. The loaded feeder is then moved from the hub facility401 to the outbound section 404, where the temporary beacon tag on thatfeeder is removed, and an appropriate entry is made in a terminalcoupled to the control system. The feeder then leaves the outboundsection 404. In this regard, if the feeder is a trailer being moved by ayardbird, it is detached from the yardbird and coupled to an availablecab, and the cab then pulls it to its destination.

[0113] As mentioned above, the system which tracks ULDs and feedersthrough the installation 400 is not shown in FIG. 10. This system isreferred to as a ULD Tracking System (UTS), and FIG. 11 is adiagrammatic view of selected portions of this UTS system, which isdesignated generally in FIG. 11 with reference numeral 500. In moredetail, the UTS system 500 includes a UTS server 502, the hardware ofwhich is a suitable computer system of a commercially available type.The server 502 is associated with a database 503, which may be stored ona hard disk of the server 502 itself, or in some type of physicallyseparate storage device that is operatively coupled to the server 502.The system 500, including the server 502, is fault tolerant in thedisclosed embodiment, including the provision of a degree of redundancy,in order to permit the system to automatically reconfigure itself in aknown manner so as to work around localized faults that may occur. Inthis regard, it will be recognized that, since all of the packages beinghandled in the installation 400 absolutely have to be delivered thefollowing day, it is simply unacceptable for a failure within the system500 to bring the operation of the installation 400 to a halt. Thetechniques used to obtain fault tolerant capability are of a known type,and are therefore not disclosed here in detail.

[0114] The server 502 is interfaced at 504 to several other systems,which technically are not part of the UTS system 500 itself, and theyare therefore shown in broken lines in FIG. 11. One is the ULD weighscale section 452, which was mentioned above in association with FIG.10. Another is an air hub control system (AHCS) 506, which is a separatecomputer system that provides overall control for the installation 400of FIG. 10, including functions other than tracking of feeders and ULDswithin the installation 400. The server 502 is also coupled to a weighand balance system 507, and an operation planning and control (OPC)system 508. The server 502 could also be optionally coupled to someother type of computer system 509 used at the facility 400.

[0115] Turning in more detail to the UTS system 500, and as mentionedabove, there are a plurality of readers which are each equivalent to thereader shown at 13 in FIG. 1. Ten of these readers are shown at 521-530in FIG. 11, but this is merely a representative sample of the totalnumber of readers provided throughout the entire installation 400. Sixreaders 521-526 from this group are each coupled to the server 502through wires of a network 536. In the disclosed embodiment, the network536 is of a type commonly known in the art as an Ethernet network. Tworeader controllers 537-538 are also coupled to the network, tofacilitate communications between the server 502 and the readers. Thestructure and operation of the reader controllers 537-538 are known tothose skilled in the art, and therefore not described here in detail.

[0116] The remaining readers 527-530 in FIG. 11 are not coupled directlyto the network 536. Instead, each is coupled to a respective wirelessreceiver/transmitter 541-544, each of which communicates throughwireless signals with a respective one of two additional wirelessreceiver transmitters 547-548, which serve as access points to thenetwork 536. These wireless links conform to a known standard which waspropagated by the Institute of Electrical and Electronic Engineers, andwhich is commonly known as the IEEE 802.11 standard. Since personsskilled in the art are already familiar with this standard, a detaileddiscussion of it is unnecessary here.

[0117] The readers 521-526 which are coupled directly to wires of thenetwork 536 are likely to be readers provided within the physicalbuilding of the hub facility 401, whereas the readers 527-530 which arecoupled to the network 536 by wireless links 541-544 and 547-548 aremore likely to be the readers which are in the inbound section 403 andthe outbound section 404. This is because the additional expense of thewireless equipment is more likely to be cost effective in exteriorlocations, where some significant cost would be involved in runningwires to isolated locations. However, the present invention does notpreclude the use of wireless links within the building of the hubfacility 401, or the use of direct network connections at locationsoutside the hub facility 401.

[0118] In FIG. 11, a choke point reader system 549 is coupled to thenetwork 536. It can cooperate with at least some of the readers 521-530,in order to provide an immediate and accurate log of the specific timeand location when a beacon tag passed a certain spot referred to as a“choke point”. A choke point is a location which many or all of thebeacon tags must pass, one example being a doorway through which allULDs must pass in order to enter the hub facility 401. The reader system549 ensures that an accurate log of the time and location is immediatelyrecorded, because the server 502 will sometimes be too busy with othertasks to respond sufficiently quickly to accurately record the time andlocation. To the extent that the choke point reader system 549 collectsinformation, it passes the information on to the server 502 in duecourse.

[0119] Server 502 is also coupled through a further network 561 and twonetwork controllers 562-563 to several wireless base stations, four ofwhich are shown at 566-569. Base stations of the type shown at 566-569are provided throughout the installation 400, and permit the server 502to communicate in a wireless manner with several wireless handhelddevices 571-578. The handheld devices 571-578 each include a keypad anda display, and are used for various purposes.

[0120] One such purpose is to permit persons throughout the facility toobtain information about a ULD, a mobile device or some other itemassociated with a given tag. The control system 14 maintains informationin an electronic form about the items associated with each tag, and canthus easily provide pertinent portions of this information on request toany of the handheld devices 571-578. Similarly, the control system couldbe configured to provide this information through the Internet to astandard “web browser” program Another purpose of the handheld devices571-578 is to permit the server 502 to issue instructions to persons whoare working within the installation 400. For example, a person operatinga mobile device such as a forklift transporting a ULD may need to begiven instructions regarding what he or she should do with the ULD. Inthis regard, if the ULD is to be taken to one of the unloading stations421 (FIG. 10), the operator needs to know which specific unloadingstation the ULD should be delivered to. Similarly, if a ULD is waitingin a staging area, and the operator is to pick it up, the operator needsto know which specific ULD to pick up. The server 502 can convey thisinformation to the operator through one of the handheld devices 571-578carried by that operator.

[0121] The handheld devices 571-578 also have the capability to functionas beacon tag readers. This permits an operator, with or without helpfrom the server 502, to identify whether a particular ULD near theoperator is a ULD which the system wants the operator to do somethingwith.

[0122] The handheld units 571-578 can also be used by an operator tonotify the server 502 of equipment which the operator is currentlyusing. For example, if the operator takes control of a yardbird in orderto move feeders around the installation 400, identification codes forthe operator and the yardbird can be entered manually on the keypad, orcan be scanned in an appropriate manner such as by scanning bar codes onthe yardbird and on the operator's badge with a bar code scanner in thehandheld device, so that server 502 knows which equipment thatparticular operator is currently using. The server 502 can then use thathandheld device to give the operator specific instructions regardingwhat the operator should do with that piece of equipment.

[0123]FIG. 12 is a diagrammatic view of a train 600, which is of a typethat has been mentioned above, and which can be used to transport ULDswithin the installation 400 of FIG. 10. The train 600 of FIG. 12includes a tractor or tug 601 which pulls the train, and three trailersor dollies 602-604. The tractor 601 and trailers 602-604 are each a typeof mobile device.

[0124] The trailers 602-604 are all identical. The trailer 602 has atits forward end a tongue, which is releasably coupled to a hitch at therear of the tractor 601. The trailer 603 has at its forward end a tonguewhich is releasably coupled to a hitch at the rear of the trailer 602,and the trailer 604 has at its forward end a tongue which is releasablycoupled to a hitch at the rear of the trailer 603. Although the train600 of FIG. 12 has three trailers, it will be recognized that the numberof trailers could be larger or smaller. Each of the trailers 602-604 hasa respective ULD 606-608 removably supported thereon. The ULDs 606-608are each identical to the ULD 381 discussed above in association withFIG. 9.

[0125] The tractor 601 has thereon a beacon tag 611, which is providedat the top of a post in order to elevate the beacon tag 611 so that isrelatively close to the signposts provided on the ceiling, one of whichis shown at 612 on a ceiling shown diagrammatically as a broken line613. The broken line circle around the signpost 612 represents thetransmission range of the signpost 612. It should be noted that thetransmission range of the signpost 612 is specifically configured sothat the trailers 602-604 will pass below the lower portion of thetransmission range of the signpost 612. Three beacon tags 616-618 areeach provided on top of a respective one of the ULDs 606-608.

[0126] As the tractor 601 moves through the installation 400, the beacontag 611 thereon will move into and out of the transmission ranges ofvarious signposts throughout the facility, thereby permitting thelocation of the tractor 601 to be accurately tracked in the mannerdescribed above in association with FIG. 7. The beacon tags 616-618provided on top of the respective ULDs 606-608 will also pass throughthe transmission ranges of various signposts, thereby facilitatingdirect and accurate tracking of the location of each of the ULDs606-608.

[0127] The tractor 601 has a signpost 623 located near the hitch on itsrear. The trailers 602-603 each have a respective signpost 626-628 on aright rear corner thereof. The trailers 602-604 also each have arespective beacon tag 631-633 supported on the tongue thereof. Asdiscussed above, the signposts on the ceiling, such as the signpost 612,each have a transmission range which ends at a height vertically abovethe trailers. Thus, the beacon tags 631-633 on the tongues of thetrailers do not pass through the transmission ranges of the signposts onthe ceiling.

[0128] The beacon tag 631 on the tongue of the trailer 602 is within thetransmission range of the signpost 623 on the rear of the tractor 601,but is outside the transmission range of the signpost 626 disposed onthe same trailer 602, because the beacon tag 631 and the signpost 626are near opposite ends of the trailer 602. Similarly, the beacon tag 632on the trailer 603 is within the transmission range of the signpost 626at the rear of the trailer 602, but is outside the transmission range ofthe signpost 627 at the rear end of the trailer 603. Further, the beacontag 633 on the trailer 604 is within the transmission range of thesignpost 627 at the rear of the trailer 603, but is outside thetransmission range of the signpost 628 which is at the rear of thetrailer 604.

[0129] With this in mind, it will be recognized that, while the tractor601 and the trailer 602 are releasably coupled to each other, the beacontag 631 on the trailer will periodically transmit a beacon signal whichincludes its own unique beacon code and which also includes the uniquesignpost code of the signpost 623 on the tractor 601. Thus, based onbeacon signals from the tag 631, the server 502 (FIG. 11) will know thatthe trailer 602 is currently coupled directly to the tractor 601.

[0130] Similarly, the beacon signals from tag 632 advise the system thatthe trailer 603 is currently coupled directly to the trailer 602. Also,the beacon signals from the tag 633 advise the system that the trailer604 is currently coupled directly to the trailer 603. With all of thisinformation, the control system knows not only that the tractor 601 andthe trailers 602-604 are all currently coupled together to form thetrain 600, but also knows the precise order in which they respectivelyappear in the train from the front to the rear. That is, the controlsystem knows that the tractor 601 precedes the trailer 602, which inturn precedes the trailer 603, which in turn precedes the trailer 604.As trains are assembled and disassembled, in order to meet the varyingneeds of the facility, the control system always has direct immediateknowledge of exactly which tractor and trailers are combined to form anyparticular train.

[0131] As discussed above in association with the ULD 381 of FIG. 9, theULDs 606-608 each have two additional beacon tags attached to a lowerportion thereof, on opposite sidewalls near diagonally opposite cornersof the bottom wall. One such beacon tag is visible in FIG. 12 on each ofthe ULDs 606-608, and these tags are respectively identified withreference numerals 641-643.

[0132] The beacon tag 641 on the ULD 606 is within the transmissionrange of the signpost 626 on the trailer 602 which carries that ULD. Thetag 641 thus transmits a beacon signal which includes its own uniquebeacon code, and also the unique signpost code for the signpost 626.Thus, the control system knows that the ULD 606 is currently supportedon the trailer 602. In a similar manner, the beacon tags 642 and 643transmit respective beacon signals which include respective signpostcodes from the signposts 627 and 628, and which respectively advise thecontrol system that the ULDs 607 and 608 are respectively supported onthe trailers 603 and 604. The beacon tags 641-643 are sufficiently lowon the ULDs 606-608 that they pass below the transmission ranges of thesignposts which are on the ceiling 613, such as the signpost 612.

[0133] If the ULD 606 had been placed on the trailer 602 with anorientation rotated 1800 about a vertical axis from the orientationshown in FIG. 12, then the beacon tag 641 would be near the front leftcorner of the trailer 602, and the third beacon tag on the ULD 606(which is not visible in FIG. 12) would be near the signpost 626 on theright rear corner of the trailer 602. That third beacon tag would thuscarry out the function of transmitting beacon signals which contain thesignpost code of signpost 626 and which advise the control system thatthe ULD 606 is currently supported on the trailer 602. With the ULD 606in this alternate position, the beacon tag 641 would be outside thetransmission ranges of the signposts 623 and 626, and thus would notinclude any signpost code in its beacon signal. Consequently, byproviding two beacon tags at diagonally opposite locations on the lowerportion of each ULD, each ULD can be placed on a trailer with either oftwo different orientations, and it is thus not necessary for employeesof the facility to be concerned about ensuring a particular orientationof each ULD when it is placed on a trailer.

[0134] The control system knows the location of the tractor 601 byvirtue of the beacon signals issued by the beacon tag 611 on the tractor601, which typically include the signpost code of one of the signpostson the ceiling, such as the signpost 612. Further, since the controlsystem also knows which trailers are currently coupled to the tractor601, and in what order, the system also knows the location of each ofthe trailers 602-604 which are coupled to the tractor 601 as the tractor601 moves through the installation shown in FIG. 10. Further, the systemknows the location of each of the ULDs 606-608 being transported by thetrain 600, not only based on the beacon signals transmitted by the tags616-618 on top of the ULDs, but also based on beacon signals transmittedby the beacon tags 641-643 on the lower portions of the ULDs, becausethe latter associate the ULDs with the train 600, and the control systemknows the location of the train.

[0135]FIG. 13 is a diagrammatic side view which shows a forklift 651that has a signpost 652 provided on its vertically movable lift. Apallet 654 is removably supported on the lift, and has a beacon tag 653provided on it. The transmission range of the signpost 652 is indicatedby a broken line circle in FIG. 13, and it will be noted that the tag653 on the pallet 654 is within this transmission range when the palletis supported on the lift. Consequently, the tag 653 will transmit beaconsignals which include its own unique beacon code and also the signpostcode of the signpost 652. Thus, the control system will know from thesebeacon signals that the pallet 654 is presently being transported by theforklift 651. If the control system knows the items which are currentlysupported on the pallet, the system will also know where those itemsare.

[0136] It would also be possible to provide beacon tags 656 and 657 oneach of the items 658 and 659 on the pallet 654. If the transmissionrange of the signpost 652 is configured so that tags 656 and 657 arewithin that transmission range, the tags 656-657 will transmitsrespective beacon signals which directly advise the system that theitems 658-659 are being transported by the forklift 651. Alternatively,the signpost 652 could be provided on the pallet 654, and the beacon tag653 could be omitted from the pallet 654. In that case, beacon signalsfrom the tags 656-657 would advise the control system of the fact thatthe items 658-659 are currently on a mobile device which is the pallet654.

[0137] The forklift 651 has a signpost 661 mounted on a post whichextends upwardly from the top of the cab. The signpost 661 transmitssignpost signals that have a transmission range which does not reach theitems 658-659 supported on the lift of the forklift 651. However, beacontags 662-664 are provided at spaced locations on the ceiling, and eachwill be within the transmission range of the signpost 661 when theforklift 651 is disposed approximately below it. Thus, based on beaconsignals from the tags 662-664, the control system can track movement ofthe forklift 651 through the facility using a technique of the typedescribed above in association with FIG. 8. As an alternative, it willbe recognized that the signpost 661 can be replaced with a beacon tag,and the beacon tags 662-664 can be replaced with signposts, in whichcase the control system would track the forklift 651 using a techniqueof the type described above in association with FIG. 7.

[0138]FIG. 14 is a diagrammatic side view showing the tail section of anairplane 671, and also a device 672 which is commonly known as a loader,and which can be used to load or unload an airplane. The airplane has agate or door 674 which has pivoted down to create an approximatelyhorizontal platform. The loader has a horizontal platform 676, and has apowered scissors support for the platform which is capable of verticallyraising and lowering the platform, so that it can be vertically alignedwith the gate 674 of the airplane.

[0139] The platform 676 of the loader 672 supports a pallet 677, and thepallet in turn supports several items, one of which is designed byreference numeral 678, Each of the items on the palette has a beacon tagon it, one of the beacon tags being indicated by reference numeral 679.The platform supports a signpost 682. In response to signpost signalsfrom the signpost 682, the tags 679 on the items 678 transmit respectivebeacon signals which advise the control system that these items are allcurrently on the loader 672 that has the signpost 682. An operator 683carries a handheld unit 684, which is equivalent to the handheld units571-574 discussed above in association with FIG. 11. Further, theoperator carries a portable signpost 686, which can be used to turn offall of the tags 679 as the items 678 are loaded onto the airplane. Thecontrol system can verify whether or not all tags have in fact beenturned off by evaluating whether any of the tags are still transmittingbeacon signals, and can provide feedback through the handheld unit 684as to whether any tags that should be off are still on. Conversely, ofcourse, if the airplane was being unloaded, the portable signpost couldbe used to turn on the tags 679, and the tags 679 would then begintransmitting respective beacon signals containing the signpost code ofsignpost 682, in order to notify the control system that the associateditems are all on the loader 672.

[0140]FIG. 15 is a diagrammatic side view of an apparatus 700 whichincludes a conveyor 702 and a signpost 703 that is stationarily mountedabove the conveyor 702 on some not-illustrated support, such as aceiling. The signpost 703 is equivalent to the signpost shown at 11 inFIG. 1. The effective transmission range of the signpost signalstransmitted by the signpost 703 is indicated by a broken line in FIG.15.

[0141] A pallet 706 is supported on the conveyor 702, and is being movedin a direction 704 by the conveyor. The pallet 706 has several items onit, one of which is designated by reference numeral 707. Each of theitems 707 is a container for packages that are subject to overnightdelivery. Each of the items 707 has on it a respective beacon tag, oneof which is indicated by reference numeral 708. Each of the beacon tagsis equivalent to the beacon tag shown at 12 in FIG. 1.

[0142] As the pallet 706 is moved in the direction 704 by the conveyor702, each of the items 707 on the pallet will pass through thetransmission range of the signpost signals from the signpost 703. Thus,each of the beacon tags 708 will transmit to a not-illustrated reader abeacon signal which includes the unique beacon code for that particularbeacon tag, and also the signpost code of the signpost 703. Thus, thecontrol system coupled to the reader will be able to determine, based onthe receipt of all these beacon signals within a certain window of time,which items 707 are presently disposed on the pallet 706. The controlsystem will also know that these items 707 and the pallet 706 arecurrently in a location where they are passing the stationary signpost703.

[0143] It would also be possible to provide a further beacon tag 711 onthe pallet 706 itself. As the pallet 706 passes the signpost 703, thetag 711 will transmit a beacon signal which includes its own uniquebeacon code, as well as the signpost code from the signpost 703, so thatthe control system knows precisely which pallet is currently passing thesignpost 703 with the items 707 supported thereon.

[0144] In some circumstances, a problem can be encountered with thearrangement shown in FIG. 15, where successive palettes are moving alongthe conveyor 702 with relatively little spacing between them. In thisregard, after the beacon tags 708 on the items 707 move out of thetransmission range of the signpost 703, they will still continue totransmit beacon signals that include the signpost code of the signpost703, for the period of time required to complete the beacon sequencewhich was discussed above in association with FIGS. 4 and 5. If anotherpallet is moving along the conveyor 702 a short distance behind theillustrated palette 706, the items on that next pallet may move into thetransmission range of the signpost 703 and begin transmitting beaconsignals with its signpost code while the beacon tags on the illustratedpallet 706 are still winding up their beacon sequences. In that case,the control system would find it difficult to distinguish which itemsare on which of the two pallets. FIG. 16 is a diagrammatic sectionalside view of an apparatus 730 which is intended to avoid this problem.

[0145] More specifically, the apparatus 730 is an alternative embodimentof the apparatus 700 shown in FIG. 15. The apparatus 730 includes all ofthe elements discussed above in association with apparatus 700. Inaddition, it includes a sensor 732 which is stationarily mounted, forexample on the same ceiling or support as the signpost 703. The sensor732 is positioned upstream of the signpost 703 with respect to thedirection 704 in which materials move along the conveyor 702. In fact,the sensor 732 is positioned so that it can detect a new pallet 706 andthe items thereon, just about the time that they first begin to moveinto the transmission range of the signpost 703. The sensor 732 may beany of several different types of known sensors, such as a sensor whichdetects the motion of the pallet 706, or a proximity sensor which sensesthe distance to the nearest item below it.

[0146] The sensor 732 is coupled by wires to the signpost 703. When thesensor 732 detects that a new pallet 706 with items 707 thereon is aboutto move into the transmission range of the signpost 703, the sensor 732sends a signal to the signpost 703, and the signpost 703 responds byaltering its signpost code. The signpost 703 could, for example,increment its signpost code. The signpost 703 could thus be assignedseveral unique and successive signpost codes which the control systemknew were all associated with a single signpost, and could successivelycycle through those codes. Alternatively, it would be possible to simplytoggle the most significant bit of the signpost code.

[0147] As the tags 708 on the new pallet move into the transmissionrange of the signpost 703, they will begin receiving signpost signalsfrom the signpost 703 that contain the modified signpost code, and theywill begin transmitting beacon signals that include their own uniquebeacon codes, and also the modified signpost code from the signpost 703.It will be recognized that, if the tags 708 on the preceding pallet haveall moved out of the transmission range of the signpost 703 before thesignpost 703 modifies its signpost code, it will be very easy for thecontrol system to distinguish the items 707 on one pallet from the items707 on the next successive pallet. However, even if the pallets arecloser than this, as illustrated in FIG. 16, such that some of the tags708 on each of the two adjacent pallets are all within the transmissionrange of the signpost 703 at the point in time when the signpost 703changes its signpost code, the control system can still accuratelydistinguish the items on one pallet from the items on another pallet.

[0148] In more detail, and as noted above, the control system will beaware of all of the possible signpost codes associated with the signpost703. Further, the beacon tags on the first pallet will each havetransmitted beacon signals that contain the prior signpost code. Ifthose beacon tags suddenly begin transmitting beacon signals with themodified signpost code, the control system can detect this and ignorethose beacon signals. In contrast, the beacon tags on the next palettewill have been sending beacon signals which do not contain any signpostcode, and will suddenly begin transmitting beacon signals which includethe modified signpost code. The control system can detect this and thusdistinguish the beacon tags on items disposed on one palette from thebeacon tags on items disposed on the other palette.

[0149] The present invention provides a number of technical advantages.One such technical advantage results from the capability to dynamicallyvary the duration of the signals transmitted by the tag. For example,the amount of information transmitted and thus the duration may beshorter when the tag is not currently receiving a signal from anysignpost, whereas the duration may be longer when the tag is receiving asignpost signal and needs to include a signpost code in the transmittedsignals. This helps reduce the cumulative amount of time during whichthe tag is actively transmitting, which in turn can facilitatecompliance with governmental regulations.

[0150] Another advantage results from the capability for the tag to varyat least one of its transmission rate and transmission power in adynamic manner, based on operating conditions such as whether the tag iscurrently receiving signals from a signpost. This also helps tofacilitate compliance with governmental regulations. Further, thesetypes of variations can help to reduce the likelihood of collisions withsignals transmitted by other tags, while ensuring that the readerreliably receives at least one transmission from each tag within areasonably short period of time.

[0151] Although several selected embodiments have been illustrated anddescribed in detail, it will be understood that other substitutions andalterations are possible without departing from the spirit and scope ofthe present invention, as defined by the following claims.

What is claimed is:
 1. An apparatus comprising a tag having circuitrywhich includes: a receiver section operable to receive wireless signpostsignals that each include a signpost code; and a transmitter sectionoperable to transmit wireless beacon signals which each include a beaconcode associated with said tag, said transmitter section being responsiveto receipt by said receiver section of a respective said signpost signalfor including in at least one said beacon signal the signpost code fromthe received signpost signal; wherein said transmitter section isoperable to transmit said beacon signals in a selected one of first andsecond formats which are different, said transmitter section using saidfirst format in response to receipt of one of said signpost signals andusing said second format in response to the absence of receipt of any ofsaid signpost signals for a specified time interval, said first formatincluding a signpost field containing the signpost code from the mostrecently received signpost signal, and said second format lacking saidsignpost field and being shorter in length than said first format.
 2. Amethod comprising the steps of: receiving in a receiver section of a tagwireless signpost signals that each include a signpost code; andtransmitting from a transmitter section of said tag wireless beaconsignals which each include a beacon code associated with said tag, saidtransmitting step including the steps of: causing said transmittersection to be responsive to receipt by said receiver section of arespective said signpost signal for including in at least one saidbeacon signal the signpost code from the received signpost signal; andcausing said transmitter section to transmit said beacon signals in aselected one of first and second formats which are different, saidtransmitter section using said first format in response to receipt ofone of said signpost signals and using said second format in response tothe absence of receipt of any of said signpost signals for a specifiedtime interval, said first format including a signpost field containingthe signpost code from the most recently received signpost signal, andsaid second format lacking said signpost field and being shorter inlength than said first format.
 3. An apparatus comprising a tag havingcircuitry which includes: a receiver section operable to receivewireless signpost signals that each include a signpost code; and atransmitter section operable to transmit wireless beacon signals whicheach include a beacon code associated with said tag, said transmittersection being responsive to receipt by said receiver section of arespective said signpost signal for including in at least one saidbeacon signal the signpost code from the received signpost signal;wherein said transmitter section is responsive to receipt by saidreceiver section of one of said signpost signals for automaticallyeffecting variation in a predetermined manner of at least one of atransmission power level and a transmission rate for said beaconsignals.
 4. An apparatus according to claim 3, wherein said transmittersection is operative to carry out said variation by: transmitting afirst series of said beacon signals containing the signpost code at afirst transmission power level; and thereafter transmitting a secondseries of said beacon signals containing the signpost code at a secondtransmission power level which is higher than said first transmissionpower level, said first series being transmitted at a effective firstrate which is substantially higher than an effective second rate atwhich said second series is transmitted.
 5. An apparatus according toclaim 4, wherein said transmission of said first series is carried outby defining a plurality of successive first time slots and transmittingeach of said beacon signals of said first series at a substantiallyrandomly selected time within a respective said first time slot; andwherein said transmission of said second series is carried out bydefining a plurality of successive second time slots and transmittingeach of said beacon signals of said second series at a substantiallyrandomly selected time within a respective said second time slot, saidsecond time slots being substantially longer than said first time slots.6. An apparatus according to claim 4, wherein said first rate is atleast ten times said second rate.
 7. An apparatus according to claim 4,wherein said transmitter section is further operative to carry out saidvariation by inhibiting transmission of said beacon signals during atime interval which occurs between transmission of said first and secondseries.
 8. An apparatus according to claim 4, wherein said transmittersection is further operative to carry out said variation bytransmitting, after said first series and before said second series, athird series of said beacon signals containing the signpost code at saidfirst transmission power level and at an effective third rate which isless than said first rate and greater than said second rate.
 9. Anapparatus according to claim 8, wherein said transmission of said thirdseries is carried out by defining a plurality of successive third timeslots and transmitting each of said beacon signals of said third seriesat a substantially randomly selected time within a respective said thirdtime slot, said third time slots being substantially longer than saidfirst time slots and substantially shorter than said second time slots.10. An apparatus according to claim 8, wherein said first rate isapproximately ten times said third rate, and said third rate isapproximately ten times said second transmission rate.
 11. An apparatusaccording to claim 8, wherein said transmitter section is furtheroperative to carry out said variation by inhibiting transmission of saidbeacon signals during each of first and second time intervals, saidfirst time interval occurring between said first and third series, andsaid second time interval occurring between said third and secondseries.
 12. A method, comprising the steps of: receiving in a receiversection of a tag wireless signpost signals that each include a signpostcode; and transmitting from a transmitter section of said tag wirelessbeacon signals which each include a beacon code associated with saidtag, said transmitting step including the steps of: causing saidtransmitter section to be responsive to receipt by said receiver sectionof a respective said signpost signal for including in at least one saidbeacon signal the signpost code from the received signpost signal; andcausing said transmitter section to be responsive to receipt by saidreceiver section of one of said signpost signals for automaticallyeffecting variation in a predetermined manner of at least one of atransmission power level and a transmission rate for said beaconsignals.
 13. A method according to claim 12, wherein said step ofautomatically effecting variation includes the steps of: transmitting afirst series of said beacon signals containing the signpost code at afirst transmission power level; and thereafter transmitting a secondseries of said beacon signals containing the signpost code at a secondtransmission power level which is higher than said first transmissionpower level, said first series being transmitted at a effective firstrate which is substantially higher than an effective second rate atwhich said second series is transmitted.
 14. A method according to claim13, wherein said step of transmitting said first series is carried outby defining a plurality of successive first time slots and transmittingeach of said beacon signals of said first series at a substantiallyrandomly selected time within a respective said first time slot; andwherein said step of transmitting said second series is carried out bydefining a plurality of successive second time slots and transmittingeach of said beacon signals of said second series at a substantiallyrandomly selected time within a respective said second time slot, saidsecond time slots being substantially longer than said first time slots.15. A method according to claim 13, including the step of selecting saidfirst rate to be at least ten times said second rate.
 16. A methodaccording to claim 13, wherein said step of automatically effectingvariation further includes the step of inhibiting transmission of saidbeacon signals during a time interval which occurs between transmissionof said first and second series.
 17. A method according to claim 13,wherein said step of automatically effecting variation further includesthe step of transmitting, after said first series and before said secondseries, a third series of said beacon signals containing the signpostcode at said first transmission power level and at an effective thirdrate which is less than said first rate and greater than said secondrate.
 18. A method according to claim 17, wherein said step oftransmitting said third series is carried out by defining a plurality ofsuccessive third time slots and transmitting each of said beacon signalsof said third series at a substantially randomly selected time within arespective said third time slot, said third time slots beingsubstantially longer than said first time slots and substantiallyshorter than said second time slots.
 19. A method according to claim 17,including the steps of selecting said first rate to be approximately tentimes said third rate, and selecting said third rate to be approximatelyten times said second transmission rate.
 20. A method according to claim17, wherein said step of automatically effecting variation furtherincludes the step of inhibiting transmission of said beacon signalsduring each of first and second time intervals, said first time intervaloccurring between said first and third series, and said second timeinterval occurring between said third and second series.